Where Castlevania set the standard for the weapons featured in the series, Castlevania II: Simon's Quest worked to expand and elaborate on hero Simon Belmont's repertoire. Every item was meant for new uses in his expanded arsenal, both for combat and exploration. It wasn't enough for him to just do damage, he had to have a purpose to what he carried.
At first glance, the Diamond (later called the "Rebound Stone" before settling on the name "Ricochet Rock") wasn't as immediately useful as the Dagger for raw damage or the Holy Water for exploration. In fact, because Simon had to use hearts both to make purchases in town and to fuel some of his sub-weapons, many players may have ignored most sub-weapons altogether (barring the very useful holy water), gathering these items and then shoving them away to be ignored.
However, the Diamond (and all other rebounding weapons to come after it) did have instances where it was very useful. It could be used to deal a lot of damage to packs in tight hallways, or to lay a field of fire against bosses. It might not have had to up front, easy to use nature of the Axe, but in the right moment it could show its worth.
About Ricochet Rock:
Ricochet Rock in Combat
First appearing in Castlevania II: Simon's Quest (gained from a gypsy in the Jam Wasteland), the Diamond could be used (for no cost) by flinging it against the ground at a 45 degree angle. It would then bounce around the screen a few times, ricocheting off walls and the ceiling, damaging any enemy it contacted. This could prove useful in many of the tight hallways of the mansions dotting the Romanian countryside, letting Simon deal quick and effective damage to any enemy in front of him.
After Simon's Quest, the diamond was shoved into the metaphorical item bag for the Castlevania series, staying hidden for ten years until it made a reappearance in Castlevania: Symphony of the Night. Renamed as the "Rebound Stone", it's function was basically the same: the heroes of that game could throw it against the ground, at a 45 degree angle, and then it would bounce around for a bit before finally disappearing.
Of note, hero Richter Belmont could item crash the Rebound Stone. This would fling four stone up into the air before a massive, area of effect wave would strike the enemies on screen. It's not really a rebound, but it was fairly effective.
And then the stone lay dormant again for a spell. It was almost another ten years before Castlevania: Portrait of Ruin brought the item back, as the Ricochet Rock, used by hero Johnathan Morris. it was functionally similar, although Johnathan would throw the rock at the ground at a 30 degree angle instead of 45 degrees. He could also master the rock, allowing him to throw two at once, one upward and one downward. They would then bounce around, as before, until they eventually dispersed.
And it was Johnathan's version that showed up once more, this time in Castlevania: Harmony of Despair. Interestingly, the game also included a new Red-type Bullet Soul for hero Soma Cruz, the Lerajie, which functioned like the Ricochet Rock.